LONDON—Major banks have suspended still-more currency traders in a sprawling probe into the guts of the foreign-exchange market, indicating that potentially questionable practices extend well beyond a single chat room, known as "The Cartel," that once appeared to be the investigation's focus.

HSBC Holdings PLC suspended two currencies traders, a spokesperson confirmed Friday—the first time since global regulators began their examination of the market last year that the bank has taken action against employees....